The present invention relates to expansion anchors for securing rock bolts in drill holes in mine roofs or other rock formations, and more specifically to an expansion anchor, and elements thereof, having novel features particularly adapted for use in relatively small diameter drill holes, either with or without a resin bonding material, with a bolt or rod of diameter previously used with larger expansion anchors.
Expansion anchors have for many years been one of the more common means employed in the support and stabilization of mine roofs and similar rock formations. Such anchors include a radially expansible shell and a tapered nut, commonly termed a camming plug, threaded onto one end of a bolt which is inserted into a drill hole in the rock formation. The diameter of the expansion shell in the unexpanded condition is very close to that of the drill hole so that the shell will frictionally engage the hole wall and be restrained from rotation as the bolt is rotated, thereby moving the camming plug axially on the bolt threads and expanding the shell into tightly gripping engagement with the hole wall. The bolt is then tensioned to a desired degree by applying the required amount of torque, forcing the bolt head, or a washer carried thereon, against a bearing plate engaging the rock surface around the entrance to the drill hole.
Another widely used means of anchoring bolts or other elongated rods, such as steel reinforcing rods ("rebar") within drill holes are cement or resin materials which fill the annular space between at least a portion of the rod and the drill hole wall and harden to bond the rod to the rock. Such materials are available in the form of two-compartment cartridges containing a resin and a catalyst which are separated from one another until insertion of the cartridge into the drill hole, at which time the rod is advanced into the hole to rupture the cartridge, release the two components, and mix them together by rotation of the rod.
Bolt anchoring systems employing both mechanical expansion anchors and resin bonding have also been proposed in various forms. In past years the amount of mixing of the resin components which resulted from insertion of the bolt and rotation thereof only for the time required to expand a conventional mechanical anchor was insufficient to provide the desired fully hardened condition of the mixture. For this reason, a number of combined resin-mechanical anchoring systems have been devised to ensure that the full advantages of both types of anchorage are realized. These include, for example, special forms of packaging to permit pre-mixing of the components while using a conventional expansion anchor, such as in U.S. Pat. No. 3,474,898 to Montgomery, and means for delaying expansion of the shell to extend the time of bolt rotation, such as in U.S. Pat. No. 3,188,815 to Schuermann et al.
Resin systems available in two-compartment cartridges at the present time, such as that marketed by DuPont under the trademark Fasloc, do not require special provisions for mixing when used in combination with expansion anchors. That is, the components are sufficiently mixed by rupturing the cartridge and forcing the components out as the bolt is inserted, and by rotation of the bolt to set a conventional expansion anchor in the usual manner, i.e., with about 3-5 seconds of rotation.
Although roof bolt expansion anchors of steel have been employed in a number of designs, the vast majority of such anchors presently in use are made of malleable iron. One of the most popular designs of malleable iron expansion anchor includes a shell having four prongs or leaves extending integrally from a closed ring at one end to opposite, free ends, with an open space or slot between adjacent leaves. The small end of the tapered nut or camming plug is inserted in the opening defined by the free ends of the leaves and is threaded on the end of a bolt extending through the expansion shell. After insertion into the drill hole, the bolt is rotated to move the camming plug axially between the leaves, the latter thus being expanded radially from their free ends, and bent outwardly from their opposite ends which remain integrally attached to the ring. The shell and plug are maintained in assembled relation on the bolt prior to use either by a support nut threaded on the bolt under the shell or by a bail having end portions engaged with the shell and extending over the plug.
Prior art successful commercial forms of these so-called four-prong shells and associated camming plugs have been produced in embodiments 11/4" or more in outside diameter by conventional casting techniques. However, smaller anchors which are simply scaled-down versions of these conventional anchors do not operate properly. One problem stems from the fact that a bolt at least 5/8" in diameter must be used, regardless of the size of the expansion anchors, so that the desired amount of tension may be applied to the bolt upon installation. Thus, while the dimensions of the shell and plug must be smaller, the plug must still be tapped with internal threads of the same diameter as in the larger anchors. Also, the threaded length of the plug cannot be substantially reduced and still maintain the required length of engagement with the bolt threads. Other problems arise from the requirements of obtaining proper bending of the leaves as they expand, providing sufficient contact area of the leaf surfaces with the bore hole wall, preventing excessive elongation or destruction of the shell leaves due to extreme compressive forces upon full installation, and ensuring adequate flow of the resin components around and through the shell in systems which combine resin and mechanical anchoring.
Accordingly, it is a principal object of the present invention to provide a malleable iron mine roof expansion anchor of the four prong type for use in a combination mechanical-resin anchor system in drill holes on the order of 1" in diameter with a 5/8" bolt.
Another object is to provide a mine roof expansion anchor having a unique combination of dimensions and details of configuration which insure proper operation and anchorage capacity in a four prong anchor having a maximum outside diameter of 1" for use with a 5/8" bolt.
A further object is to provide a malleable iron mine roof expansion anchor for use in a drill hole slightly over 1" in diameter with a 5/8" bolt, and having dimensional and constructional features which insure proper operation and holding capacity when used both with and without supplementary resin anchoring.
Still another object is to provide a novel tapered camming plug for a mine roof expansion anchor having constructional features particularly useful in relatively small scale versions of such anchors, and when used in conjunction with resin anchoring.
A still further object is to provide a novel leaf configuration for incorporation in a four-prong malleable iron expansion anchor shell having a maximum external diameter of not more than 1" and properly operable with a roof bolt 5/8" in diameter.
Other objects will in part be obvious and will in part appear hereinafter.